During construction of oil and gas wells, a rotary drill is typically used to bore through subterranean formations of the earth to form a borehole. As the rotary drill bores through the earth, a drilling fluid, known in the industry as a “mud,” is circulated through the borehole. Drilling fluids are usually pumped from the surface through the interior of the drill pipe. By continuously pumping the drilling fluid through the drill pipe, the drilling fluid can be circulated out the bottom of the drill pipe and back up to the well surface through the annular space between the wall of the well bore and the drill pipe. The hydrostatic pressure created by the column of mud in the hole prevents blowouts which would otherwise occur due to the high pressures encountered within the well. The drilling fluid is also used to help lubricate and cool the drill bit and facilitates the removal of cuttings as the borehole is drilled.
Once the well bore has been drilled, casing is lowered into the well bore. A cement slurry is then pumped into the casing and a plug of fluid, such as drilling mud or water, is then pumped behind the cement slurry in order to force the cement up into the annulus between the exterior of the casing and the borehole. The cement slurry is then allowed to set and harden to hold the casing in place. Very low cement compressive strength is required for this purpose; the required compressive strength being dependent on casing and hole diameter. Generally, a compressive strength of 500 psi is sufficient for any combination of hole/casing for a typical oil well.
The cement also provides zonal isolation of the subsurface formations, helps to prevent sloughing or erosion of the well bore and protects the well casing from corrosion from fluids which exist within the well. In this scenario the important factor is the final permeability of the set cement, which is strictly related to the solid content of the slurry and consequently to the compressive strength of the set cement. Thus, to prevent fluid movements, the cement should produce a permeability lower than 0.05 milliDarcies. To achieve this, the minimum water content in the slurry is no greater than around 70% by weight of cement, preferably between about 37% to about 50%. Such water cement ratios typically render a compressive strength higher than 1000 psi/48 hours and usually higher than 2000 PSI in 48 hours depending on the type of cement, curing temperature and other additional components of the slurry.
Typically, hydraulically-active cementitious materials, particularly Portland cements, are used to cement the well casing within the well bore. Hydraulically-active cementitious materials set and develop compressive strength due to the occurrence of a hydration reaction which allows them to set or cure under water. The physical properties of the set cement relate to the crystalline structure of the calcium-silicate-hydrates formed during hydration. For example, conventional Portland cements form an interlocking crystalline network of, for example, tricalcium silicate, dicalcium silicate, tetracalcium aluminum ferrite and calcium hydroxide crystals. These crystals interconnect to form an interlocking crystalline structure which provides both flexural strength and a degree of resiliency.
Typical cement compositions used in the prior art have a flexural strength to compressive strength ratio (FS/CS) of about 0.1 to about 0.25. The strength and durability of the crystalline structure depends largely on the water to cement ratio, porosity of hard set cement to the extent the pores are interconnected, i.e., to what degree permeability is developed.
While the development of cement compositions exhibiting higher flexural strength are desired, it further is desired to develop cement compositions which do not display a corresponding increase in compressive strength. The ability to decouple the flexural strength from compressive strength has applications where high compressive strengths are often not desirable due to low well bore stress conditions or where low compressive strength cements are unable to withstand high well bore stresses, such as deviated wells, gas wells, geothermal wells, steam injection wells and deep wells.